Evaluation of deformable image coregistration in adaptive dose painting by numbers for head-and-neck cancer.

PURPOSE

To assess the accuracy of contour deformation and feasibility of dose summation applying deformable image coregistration in adaptive dose painting by numbers (DPBN) for head and neck cancer.

METHODS AND MATERIALS

Data of 12 head-and-neck-cancer patients treated within a Phase I trial on adaptive (18)F-FDG positron emission tomography (PET)-guided DPBN were used. Each patient had two DPBN treatment plans: the initial plan was based on a pretreatment PET/CT scan; the second adapted plan was based on a PET/CT scan acquired after 8 fractions. The median prescription dose to the dose-painted volume was 30 Gy for both DPBN plans. To obtain deformed contours and dose distributions, pretreatment CT was deformed to per-treatment CT using deformable image coregistration. Deformed contours of regions of interest (ROI(def)) were visually inspected and, if necessary, adjusted (ROI(def_ad)) and both compared with manually redrawn ROIs (ROI(m)) using Jaccard (JI) and overlap indices (OI). Dose summation was done on the ROI(m), ROI(def_ad), or their unions with the ROI(def).

RESULTS

Almost all deformed ROIs were adjusted. The largest adjustment was made in patients with substantially regressing tumors: ROI(def) = 11.8 ± 10.9 cm(3) vs. ROI(def_ad) = 5.9 ± 7.8 cm(3) vs. ROI(m) = 7.7 ± 7.2 cm(3) (p = 0.57). The swallowing structures were the most frequently adjusted ROIs with the lowest indices for the upper esophageal sphincter: JI = 0.3 (ROI(def)) and 0.4 (ROI(def_ad)); OI = 0.5 (both ROIs). The mandible needed the least adjustment with the highest indices: JI = 0.8 (both ROIs), OI = 0.9 (ROI(def)), and 1.0 (ROI(def_ad)). Summed doses differed non-significantly. There was a trend of higher doses in the targets and lower doses in the spinal cord when doses were summed on unions.

CONCLUSION

Visual inspection and adjustment were necessary for most ROIs. Fast automatic ROI propagation followed by user-driven adjustment appears to be more efficient than labor-intensive de novo drawing. Dose summation using deformable image coregistration was feasible. Biological uncertainties of dose summation strategies warrant further investigation.